1. Field of the Invention
The present invention relates to a sliding member made of a silicon nitride ceramic and provided with a sliding surface, examples of which member include engine valve train parts such as a cam follower and a valve, a fuel injection pump part, and a bearing; and a process for producing the same.
2. Description of the Prior Art
Industrial equipment includes a large number of mutually sliding portions of machine parts. The generation of the loss of energy in the sliding portions and the wear thereof causes a decrease in energy efficiency and a decrease in the life span of the equipment. In view of this, improvement in the accuracy of sliding surfaces and development of novel sliding member materials have been under way in order to decrease the friction resistance in sliding portions and improve the durability of sliding members.
Ceramics have attracted attention as such novel sliding member materials. Silicon nitride (Si.sub.3 N.sub.4) in particular is hoped for as a material of sliding members since it is excellent in strength and wear resistance. As for utilization of a silicon nitride ceramic in sliding members, there have been proposed utilization thereof in a cam follower as a car engine sliding part (see Japanese Patent Laid-Open No. 65,809/1993), and utilization thereof in an exhaust valve (see Japanese Patent Laid-Open No. 24,073/1989) in addition to utilization thereof in a ball bearing, a plain bearing, etc.
Although a silicon nitride sintered body once obtained through sintering is machined into a predetermined shape while using a diamond grind wheel or the like to produce a complicatedly shaped member such as a cam follower or an exhaust valve, the machining time is long with a large amount of machining due to the poor machinability of the silicon nitride sintered body. Accordingly, the fact is that the member thus obtained is very expensive to hinder the wide practical use thereof.
In view of the foregoing, various processes for producing a high-precision powder compact as attempts to improve the dimensional accuracy of a silicon nitride sintered body in order to decrease the amount of grinding of the silicon nitride sintered body have been under investigation. However, the dimensional accuracy and configurational accuracy of a sintered body obtained even according to a process for improving the dimensional accuracy of a powder compact cannot be said to be satisfactory.
Under such circumstances, attempts to produce a complicatedly shaped member through superplastic working of a silicon nitride ceramic have been made as proposed, for example, in Japanese Patent Publication No. 68,427/1993 and Japanese Patent Laid-Open No. 103,303/1992. According to such superplastic working, however, a composite material of silicon nitride and silicon carbide is the object of working, and Si.sub.3 N.sub.4 grains before and after superplastic working must be granular crystals.
According to the above-mentioned superplastic working, Si.sub.3 N.sub.4 must be admixed and composited with SiC to entail a difficulty in obtaining a dense sintered body, resulting in a failure in securing high mechanical strength and reliability thereof. In view of this, Japanese Patent Laid-Open No. 103,303/1992 discloses an attempt to grow Si.sub.3 N.sub.4 grains into columnar crystals through a heat treatment of the former after superplastic working thereof to thereby improve the flexural strength of a sintered body. However, the heat treatment must be effected for a long period of time, and moreover involves a dimensional or configurational change of the sintered body to further require grinding or the like for correction thereof.
Another attempt to effect superplastic working of a silicon nitride sintered body comprising Si.sub.3 N.sub.4 as the main component but not containing SiC has been made, which however requires either long-time superplastic working due to poor workability thereof or use of a sintered body having a very fine texture to entail many industrial problems.
U.S. Pat. No. 4,732,719 discloses an attempt to densify the texture of Si.sub.3 N.sub.4 during superplastic working by admixing and compositing Si.sub.3 N.sub.4 with Si. However, this entails problems such as a low strength of the resulting sintered body because Si remains in the sintered body.